1. Field of the Invention
The present invention relates to a radio frequency sensor structure, more particularly, to an active radio frequency sensor structure having a self-mixing and self-demodulation architecture.
2. Description of the Related Art
From ancient times, the continued development of means of transportation has been one of the driving forces for human civilization. The evolution of various means of transportation illustrates the necessary role that they have played in pushing forward human civilization. As technology continues to progress, nowadays vehicle performance gets better and better, human mobility becomes faster and faster, and more and more road conditions dynamically occur. A variety of different automotive electronics products having different road traffic monitoring functions emerge.
The conventional microwave/radio frequency sensor is to independently design the antenna and the transceiver module. Please refer to FIG. 1, FIG. 1 is a schematic diagram showing a structure of a microwave radio frequency sensor according to the prior art. The prior art microwave/radio frequency sensor 10 comprises a demodulation circuit 11, a voltage controlled oscillator 12, a power divider 13, a driving amplifier 14, a transmitting antenna 15, a receiving antenna 16, a low noise amplifier 17, and a mixer 18. The demodulation circuit 11 is used for generating the required demodulated signal, and emitting the demodulated signal to the voltage controlled oscillator 12. The voltage controlled oscillator 12 is an electronic oscillating circuit which controls the oscillation frequency in accordance with the input voltage and finally outputs the frequency modulated continuous wave. The transmitting antenna 15 then emits a transmitted wave to the road. However, before the frequency modulated continuous wave passing the transmitting antenna 15, the power divider 13 will input a portion of the power output from the voltage controlled oscillator 12 to the mixer 18 so as to obtain an intermediate frequency signal. The driving amplifier 14 will amplify the frequency modulated continuous wave before the transmitting antenna 15 receives the frequency modulated continuous wave.
The receiving antenna 16 will input the received signal to the mixer 18 after receiving the echo signal. Before the signal is transmitted to the mixer 18, the low noise amplifier 17 will not only amplify the signal but also inhibit noise to allow subsequent electronic components performing signal processing. The mixer 18 then calculates the frequency difference between the transmitted wave and the echo wave, down converts the frequency difference and outputs the intermediate frequency signal. After that, the back-end intermediate frequency circuit (not shown in the figure) regulates the range of detection distance (range measurement) to obtain the information of the detection target. Finally, the analog signal is converted to the digital signal and sent to the digital signal processor (not shown in the figure). The object recognition is performed to achieve the objective of traffic monitoring.
As mentioned previously, the antenna and the radio frequency circuit module are first independently designed then integrated according to the prior art. A dual-antenna architecture is generally adopted in system design so that the isolation between the transmitting path and the receiving path is increased. When considering the size of system, the architecture having a single antenna cooperating with a circulator is adopted to isolate the transmitting path from the receiving path. Nevertheless, the prior art has its limitation in applications. When the prior art is applied to low-cost, small-sized (diameter is smaller than 2 cm), and low power consumption (power consumption is less than 1.5 W) products that are used for detecting moving objects at a short distance (within 20 m) and having a wide coverage angle (approximately 360 degree, that is omnidirectional), the following problems are encountered.
Both the above-mentioned dual-antenna and single-antenna architectures perform detecting based on standard radar principle. In other words, the power level is a major reference for the range measurement. At the same time, the size of the antenna is directly related to the operating frequency. When the operating frequency is lower, the antenna size is larger but the 3 dB beamwidth is larger to result in a wide detection range. Conversely, when the operating frequency is higher, the antenna size is smaller but the 3 dB beamwidth is smaller to result in a narrow detection range. Because the adoption of dual-antenna would increase the volume, it is very inappropriate to typical small-sized sensors when the dual-antenna architecture is utilized. But if the carrier frequency is increased, the 3 dB beamwidth of the antenna becomes smaller, which is inappropriate to the detection of moving objects in a wide-angle environment. When the single-antenna architecture is adopted, although its volume is smaller than that of the dual-antenna architecture, its cost is higher than that of the dual-antenna architecture owing to the implementing of the circulator. Moreover, the highest reverse path isolation is only 35 dB that is significantly lower than that of the dual-antenna architecture. In addition, it is impossible to add a power amplifier or other unidirectional element at the transmitting path.
It is therefore very important to design a new active radio frequency sensor structure that integrates the antenna and the transceiver circuit together so as to satisfy the demand for miniaturization.